Replacement gates to enhance transistor strain

ABSTRACT

Some embodiments of the present invention include apparatuses and methods relating to NMOS and PMOS transistor strain.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/305,465, entitled “REPLACEMENT GATES TO ENHANCE TRANSISTORSTRAIN” filed on Dec. 16, 2005 and claims priority there from.

TECHNICAL FIELD

Embodiments of the invention relate to semiconductor technology. Inparticular, embodiments of the invention relate to strained transistors.

BACKGROUND

In semiconductor processing, transistors may be formed on semiconductorwafers. The transistors may include a gate structure having a gatedielectric and a gate electrode, a source, a drain, and a channel regionbetween the source and the drain. In CMOS (complimentary metal oxidesemiconductor) technology, transistors may typically be of two types:NMOS (negative channel metal oxide semiconductor) or PMOS (positivechannel metal oxide semiconductor) transistors. The transistors andother devices may be interconnected to form integrated circuits (ICs)which perform numerous useful functions.

The performance of the ICs may directly depend on the performance of thetransistors. Transistor performance may be improved by providing astrain in the channel region. Specifically, NMOS transistor performancemay be improved by providing a tensile strain in the channel region andPMOS transistor performance may be improved by providing a compressivestrain in the channel region.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by way of example and not by way oflimitation in the figures of the accompanying drawings, in which thelike references indicate similar elements and in which:

FIG. 1 is a cross-sectional view of a partially formed NMOS transistorand a partially formed PMOS transistor, and a layer over the transistorgate structures.

FIG. 2 is a view similar to FIG. 1 with a portion of the layer removedto expose the gate structures.

FIG. 3 is a view similar to FIG. 2 with the transistor gate structuresremoved to form trenches.

FIG. 4 is a view similar to FIG. 3 with a gate dielectric formed in thetrenches.

FIG. 5 is a view similar to FIG. 4 with gate electrodes formed in thetrenches.

FIG. 6 is a view similar to FIG. 4 with an n-type material formed in thetrenches.

FIG. 7 is a view similar to FIG. 6 with a portion of the n-type materialremoved and a p-type material formed in the trenches.

FIG. 8 is a view similar to FIG. 7 with fill materials formed in thetrenches.

FIG. 9 is a view similar to FIG. 2 with a portion of the gate structuresremoved to form trenches.

FIG. 10 is a view similar to FIG. 9 with gate electrodes formed in thetrenches.

DETAILED DESCRIPTION

In various embodiments, apparatuses and methods relating to strainedtransistors are described. However, various embodiments may be practicedwithout one or more of the specific details, or with other methods,materials, or components. In other instances, well-known structures,materials, or operations are not shown or described in detail to avoidobscuring aspects of various embodiments of the invention. Similarly,for purposes of explanation, specific numbers, materials, andconfigurations are set forth in order to provide a thoroughunderstanding of the invention. Nevertheless, the invention may bepracticed without specific details. Furthermore, it is understood thatthe various embodiments shown in the figures are illustrativerepresentations and are not necessarily drawn to scale.

Transistor performance characteristics, such as transistor drivecurrent, may be enhanced by providing strain to the transistor channels.For example, NMOS transistor performance may improve when the NMOStransistor channel is under lateral tensile stress. Also, PMOStransistor performance may improve when the PMOS transistor channel isunder lateral compressive stress. Briefly, the present invention mayprovide for increased channel strain to enhance the performance of NMOSand PMOS transistors.

FIG. 1 illustrates an NMOS transistor 101 and a PMOS transistor 102 on asubstrate 103. NMOS transistor 101 and PMOS transistor 102 may be on acontinuous substrate 103. However, they are illustrated separately inFIGS. 1-10 for the sake of clarity. Substrate 103 may include anysuitable semiconductor material. In an embodiment, substrate 103 mayinclude monocrystalline silicon. Substrate 103 may also includeisolation structures (not shown) to isolate the NMOS and PMOStransistors.

As shown, PMOS transistor 102 includes an n-well 111, a channel 112,source and drain films 113, a gate dielectric 114, a gate electrode 115and spacers 116. PMOS transistor 102 may be formed by any suitableprocessing techniques.

N-well 111 may include any suitable n-type dopants, such as phosphorusand arsenic, and may be formed by any suitable technique. In anembodiment, n-well 111 may be formed by doping substrate 103 by ionimplantation.

Channel 112 may be under a compressive strain from any stressor sourceor material. As discussed, a compressive strain may increase theperformance of PMOS transistor 102. In an embodiment, source and drainfilms 113 may provide a compressive strain on channel 112. In suchembodiments, source and drain films 113 may therefore be consideredstressors. In an embodiment, source and drain films 113 may be epitaxialand may have a greater lattice spacing constant than n-well 111 andchannel 112. Source and drain films 113 may transfer a compressivestrain on channel 112 as they tend to, but are constrained from,expanding to their natural lattice spacing. In an embodiment, source anddrain films 113 may include an alloy of materials. In an embodiment,source and drain films 113 may include an alloy of silicon andgermanium. In an embodiment, source and drain films 113 may include ap-type dopant, such as boron. In an embodiment, source and drain films113 may be formed in recesses of n-well 111.

As illustrated, channel 112 may be under a compressive strain fromsource and drain films 113. However, channel 112 may be under acompressive strain from any suitable stressor source or material. In anembodiment, a material may be formed over gate electrode 115 and spacers116 to provide a compressive strain on channel 112.

Gate dielectric 114 may be any suitable material. In an embodiment, gatedielectric 114 may include silicon dioxide. In other embodiments, gatedielectric 114 may include a high-k gate dielectric. In general, ahigh-k gate dielectric may include any material having a dielectricconstant, k, that is greater than about 3.9 (the dielectric constant ofsilicon dioxide). In an embodiment, gate dielectric 114 may includehafnium oxide. In other embodiments, gate dielectric 114 may includehafnium silicon oxide, lanthanum oxide, zirconium oxide, zirconiumsilicon oxide, tantalum oxide, titanium oxide, barium strontium titaniumoxide, barium titanium oxide, strontium titanium oxide, yttrium oxidealuminum oxide, lead scandium tantalum oxide, or lead zinc niobate.

Gate electrode 115 may include any suitable material. In an embodiment,gate electrode 115 may include polysilicon. In another embodiment, gateelectrode 115 may include polysilicon doped with a p-type dopant, suchas boron.

Gate dielectric 114 and gate electrode 115 together may be considered agate stack or a gate structure. In an embodiment, both gate dielectric114 and gate electrode 115 may be a part of the gate structure. In otherembodiments, the gate structure may be a single material. In otherembodiments, the gate structure may include a hard mask or any number ofadditional materials. As is described further below, a portion or anentirety of the gate structure may be removed to enhance the strain inchannel 112. Therefore, a portion or an entirety of the gate structuremay be considered sacrificial.

Spacers 116 may include any suitable dielectric materials, such as anitride or an oxide. Spacers 116 may be along the sidewalls of gateelectrode 108 and may therefore be considered sidewall spacers.

As shown, NMOS transistor 101 includes a p-well 104, a channel 105,source and drain regions 106, a gate dielectric 107, a gate electrode108, and spacers 109. NMOS transistor 101 may be formed by any suitableprocessing techniques. P-well 104 may include any suitable p-typedopants, such as boron and indium, and may be formed by any suitabletechnique. Source and drain regions 106 may include any suitable n-typedopants, such as phosphorus and arsenic, and may be formed by anysuitable technique, such as ion implantation or epitaxial deposition.Channel 105 may be between source and drain regions 106.

Gate dielectric 107 may be any suitable material. In an embodiment, gatedielectric 107 may include silicon dioxide. In other embodiments, gatedielectric 107 may include a high-k gate dielectric. In an embodiment,gate dielectric 107 may include hafnium oxide. In other embodiments,gate dielectric 107 may include hafnium silicon oxide, lanthanum oxide,zirconium oxide, zirconium silicon oxide, tantalum oxide, titaniumoxide, barium strontium titanium oxide, barium titanium oxide, strontiumtitanium oxide, yttrium oxide aluminum oxide, lead scandium tantalumoxide, or lead zinc niobate. In an embodiment, gate dielectric 107 andgate dielectric 114 may include the same material.

Gate electrode 108 may include any suitable material. In an embodiment,gate electrode 108 may include polysilicon. In another embodiment, gateelectrode 108 may include polysilicon doped with an n-type dopant, suchas phosphorus and arsenic.

Gate dielectric 107 and gate electrode 108 together may be considered agate stack or a gate structure. In an embodiment, both gate dielectric107 and gate electrode 108 may be a part of the gate structure. In otherembodiments, the gate structure may be a single material. In otherembodiments, the gate structure may include a hard mask or any number ofadditional materials. As is described further below, a portion or anentirety of the gate structure may be removed to enhance the strain inchannel 105. Therefore, a portion or an entirety of the gate structuremay be considered sacrificial.

Spacers 109 may include any suitable dielectric materials, such as anitride or an oxide. Spacers 109 may be along the sidewalls of gateelectrode 108 and may therefore be considered sidewall spacers.

Channel 105 may be under a tensile strain from any stressor source. Asdiscussed, a tensile strain in channel 105 may improve the performanceof NMOS transistor 101. In an embodiment, a layer 110 may cover NMOStransistor 101 and PMOS transistor 102 and layer 110 may provide atensile stress on channel 105 and channel 112 and may therefore beconsidered a tensile layer or a stressor. In an embodiment, layer 110may include a silicon nitride, such as Si₃N₄. Layer 110 may be formed byany suitable technique. In an embodiment, a tensile strain in channel112 may diminish the performance of PMOS transistor 102; however, due tosource and drain films 113, channel 112 may be under a net compressivestrain.

As discussed, channel 105 may be under a tensile strain due to layer110. However, channel 105 may be under a tensile strain from anysuitable stressor material or source. In an embodiment, source and drainregions 106 may provide a tensile strain on channel 105. In anembodiment, source and drain regions 106 may be epitaxial and may have asmaller lattice spacing constant than p-well 104 and channel 105. Sourceand drain regions 106 may then be constrained from achieving theirnatural lattice spacing and may transfer a tensile strain on channel 105as they tend to, but are constrained from, contracting to their naturallattice spacing. In an embodiment, source and drain regions 106 filmsmay include carbon.

NMOS transistor 101 and PMOS transistor 102 may also include otherfeatures that are not shown for the sake of clarity, such as haloimplants, tip implants, silicide regions, and the like.

As illustrated in FIG. 2, a portion of layer 110 may be removed toexpose gate electrode 108 and gate electrode 115. In an embodiment, aportion of layer 110 may be removed by a planarization or chemicalmechanical polishing (CMP) process. In an embodiment, a portion of gateelectrodes 108, 115 may also be removed.

As illustrated in FIG. 3, gate electrodes 108, 115 and gate dielectrics107, 114 may be removed to form trenches 121, 122. Gate electrodes 108,115 and gate dielectrics 107, 114 may be removed by any suitabletechnique, such as a selective etch technique.

As discussed, channel 105 may be under a tensile strain and channel 112may be under a compressive strain. Due to the removal of gate electrodes108, 115 and gate dielectrics 107, 114, the strain on channels 105, 112may be increased or enhanced. The increased strain may be due to theremoval of material from trenches 121, 122, which may have beenoffsetting the desired stresses. In general, removing a portion or theentirety of a gate structure may remove an offsetting stress and allowthe stressor to relax, which may increase the strain on the transistorchannel. In other words, removing the gate structure may allow thestressor to enhance the desired strain on the channel.

In an embodiment, NMOS transistor 101 may include a stressor that isrelaxed to increase a tensile strain on channel 105. In an embodiment,the stressor may include layer 110. In another embodiment, the stressormay include source and drain regions 106. In an embodiment, PMOStransistor 102 may include a stressor that is allowed to increase acompressive strain on channel 112. In an embodiment, the stressor mayinclude source and drain films 113.

As illustrated in FIG. 4, a gate dielectric 131 may be formed intrenches 121, 122. Gate dielectric 131 may be any suitable material andmay be formed by any suitable technique. In an embodiment, gatedielectric 131 may include a high-k gate dielectric, such as hafniumoxide. In various other embodiments, gate dielectric 131 may includehafnium silicon oxide, lanthanum oxide, zirconium oxide, zirconiumsilicon oxide, tantalum oxide, titanium oxide, barium strontium titaniumoxide, barium titanium oxide, strontium titanium oxide, yttrium oxidealuminum oxide, lead scandium tantalum oxide, or lead zinc niobate. Inanother embodiment, gate dielectric 131 may include silicon dioxide.

In an embodiment, gate dielectric 131 may be formed by a depositionprocess, such as chemical vapor deposition (CVD). As illustrated in FIG.4, gate dielectric 131 may be formed along the bottom of trenches 121,122 and along the sidewalls of trenches 121, 122. In an embodiment, gatedielectric 131 may also be formed over layer 110 (not shown). In suchembodiments, gate dielectric 131 may be subsequently removed from layer110 by a CMP process such that gate dielectric 131 only remains intrenches 121, 122.

As illustrated in FIG. 5, a gate electrode 141 and a gate electrode 142may be formed. Gate electrode 141 may include any suitable material ormaterials. In an embodiment, gate electrode 141 may include an n-dopedpolysilicon. In another embodiment, gate electrode 141 may include ametal with an n-type work function. In other embodiments, gate electrode141 may include a stack or structure of materials with the material incontact with gate dielectric 131 including a material with an n-typework function. In various embodiments, the n-type work function materialmay include hafnium, zirconium, titanium, tantalum, aluminum, theiralloys, or carbides of those metals.

In an embodiment, gate electrode 141 may include a material thatprovides an additional tensile strain on channel 105. Gate electrode 141may provide an additional tensile strain on channel 105 by including amaterial with a coefficient of thermal expansion (CTE) less than the CTEof substrate 103 that is deposited at a temperature greater than roomtemperature and the operating temperature of NMOS transistor 101. Uponcooling, gate electrode 141 may contract more slowly than thesurrounding materials and gate electrode 141 may then transfer a tensilestrain to channel 105 via gate dielectric 131 or via spacers 109. In anembodiment, the material that provides the additional tensile strain maybe the n-type work function metal, as listed above. In otherembodiments, the material that provides the additional tensile strainmay be another material in the gate structure. In an embodiment, thematerial that provides the additional tensile strain may includetungsten. In another embodiment, the material that provides theadditional tensile strain may include titanium carbide.

Gate electrode 142 may include any suitable material or materials. In anembodiment, gate electrode 142 may include a p-doped polysilicon. Inanother embodiment, gate electrode 142 may include a metal with a p-typework function. In other embodiments, gate electrode 142 may include astack or structure of various materials with the material in contactwith gate dielectric 131 including a material with a p-type workfunction. In various embodiments, the p-type work function material mayinclude ruthenium, palladium, platinum, cobalt, nickel, and theiroxides.

In an embodiment, gate electrode 142 may include a metal that providesan additional compressive strain on channel 112. Gate electrode 142 mayprovide an additional compressive strain on channel 112 by including amaterial with a CTE greater than the CTE of substrate 103 that isdeposited at a temperature greater than room temperature and theoperating temperature of PMOS transistor 102. Upon cooling, gateelectrode 142 may contract more quickly than the surrounding materialsand gate electrode 142 may then transfer a compressive strain to channel112 via gate dielectric 131 or via spacers 116. In an embodiment, thematerial that provides the additional compressive strain may be thep-type work function metal, as listed above. In other embodiments, thematerial that provides the additional compressive strain may be anothermaterial in the gate structure. In various embodiments, the materialthat provides the additional compressive strain may include boroncarbide, tungsten, molybdenum, rhodium, vanadium, platinum, ruthenium,beryllium, palladium, cobalt, titanium, nickel, copper, tin, aluminum,lead, zinc, alloys of these metals, or their silicides.

Gate electrode 141 and gate electrode 142 may be formed by any suitabletechnique. In an embodiment, gate electrode 141 may be selectivelyformed and then gate electrode 142 may be selectively formed. In anembodiment, gate electrode 142 may be selectively formed and then gateelectrode 141 may be selectively formed.

FIGS. 6-8 illustrate a method for forming gate electrodes with referenceto the illustration of FIG. 4. As illustrated in FIG. 6, an n-typematerial 161 may be formed in trenches 121, 122. N-type material 161 maybe any suitable n-type work function material as discussed above withreference to FIG. 5. In an embodiment, n-type material 161 may partiallyfill trenches 121, 122. In another embodiment, n-type material 161 maycompletely fill trenches 121, 122.

As illustrated in FIG. 7, n-type material 161 may be selectively removedfrom PMOS transistor 102 and a p-type material 162 may be formed. P-typematerial 162 may be any suitable p-type work function material asdiscussed above with reference to FIG. 5. In an embodiment, p-typematerial 162 may be formed such that an opening remains in either orboth of the trenches. In another embodiment, p-type material 162 maycomplete the gate structure of PMOS transistor 102 or NMOS transistor101.

As illustrated in FIG. 8, fill material 171 and fill material 172 may beselectively formed. Fill materials 171, 172 may be the same material orthey may be different. In an embodiment, fill material 171 and fillmaterial 172 may be chosen based on their conductive properties. Inanother embodiment, fill material 171 may be chosen to provide a tensilestrain on NMOS transistor 101 as discussed with respect to FIG. 5. Invarious embodiments, fill material 171 may include tungsten or titaniumcarbide.

Fill material 172 may also include any suitable material. In anembodiment, fill material 172 may be chosen to provide a compressivestrain on PMOS transistor 102 as discussed with respect to FIG. 5. Insome embodiments, fill material 172 may include boron carbide, tungsten,molybdenum, rhodium, vanadium, platinum, ruthenium, beryllium,palladium, cobalt, titanium, nickel, copper, tin, aluminum, lead, zinc,alloys of these metals, or their silicides.

As illustrated in FIGS. 6-8, n-type material 161 may be formed andselectively removed, p-type material 162 may be formed, and optionalfill materials may be formed. In another embodiment, the n-type andp-type materials may be formed in the opposite order. For example, thep-type material may be formed first. Then, the p-type material may beselectively removed from NMOS transistor 101 and a subsequent n-typematerial may then be formed. Lastly, optional fill materials may then beformed.

The illustrated methods of FIGS. 3-8 show trenches 121, 122 being formedand the stressors on channels 105, 112 being allowed to simultaneouslyincrease the strain on channels 105, 112. However, in other embodiments,trenches 121, 122 may be formed and the strains increased independently.In an embodiment, trench 121 may be formed while gate dielectric 114 andgate electrode 115 remain. The strain on channel 105 of NMOS transistor101 may then be independently increased. In another embodiment, trench122 may be formed while gate dielectric 107 and gate electrode 108remain. The strain on channel 112 of PMOS transistor 102 may then beindependently increased.

The methods illustrated in FIGS. 3-8 show the entire gate structurebeing sacrificial. As illustrated in FIGS. 9-10, only a portion of thegate structure may be sacrificial.

In FIG. 9, gate electrodes 108, 115 may be removed from the structure ofFIG. 2 to form trenches 151, 152. Gate electrodes 108, 115 may beremoved by any suitable technique such as a selective etch technique.

Due to the removal of gate electrodes 108, 115 the strain on channels105, 112 may be increased. The increased strain may be due to theremoval of material from trenches 151, 152, which may have been anoffsetting stress to the desired stresses as discussed above.

As illustrated in FIG. 10, gate electrodes 141, 142 may be formed. Gateelectrodes 141, 142 may include any of the materials or attributes andmay be formed by any of the methods as discussed above with respect toFIGS. 5-8.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, material, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the invention. Thus, the appearances ofthe phrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily referring to the sameembodiment of the invention. Furthermore, the particular features,structures, materials, or characteristics may be combined in anysuitable manner in one or more embodiments.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of ordinary skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A method comprising: forming an NMOS transistor including a channel,a stressor, and a gate structure over the channel and between sidewallspacers, wherein the stressor causes a tensile strain on the channel;and removing at least a portion of the gate structure to allow thestressor to enhance the tensile strain on the channel, wherein removingthe portion of the gate structure forms a trench.
 2. The method of claim1, wherein the stressor comprises a tensile layer over the gatestructure.
 3. The method of claim 1, wherein removing the portion of thegate structure includes removing a gate electrode, and wherein a gatedielectric remains.
 4. The method of claim 3, wherein the gatedielectric comprises a high-k gate dielectric.
 5. The method of claim 4,further comprising: forming a metal gate electrode over the high-k gatedielectric.
 6. A method comprising: forming a PMOS transistor includinga channel, a stressor, and a gate structure over the channel and betweensidewall spacers, wherein the stressor causes a compressive strain onthe channel; and removing at least a portion of the gate structure toallow the stressor to enhance the compressive strain on the channel,wherein removing the portion of the gate structure forms a trench. 7.The method of claim 6, wherein the stressor comprises an epitaxialsource and drain film.
 8. The method of claim 6, wherein removing theportion of the gate structure includes removing a gate electrode and agate dielectric.
 9. The method of claim 8, further comprising: forming ahigh-k gate dielectric in the trench; and forming a metal gate electrodeover the high-k gate dielectric, wherein the metal gate electrodeprovides an additional compressive strain in the channel.